Electrohydraulic servovalve having minimum flow during quiescent operation



June 25, 1968 VAN AUSDAL ET AL 3,389,719

ELECTROHYDRAULIC SERVOVALVE HAVING MINIMUM FLOW DURING QUIESCENT OPERATION 2 Sheets-Sheet 1 Filed 0st. 18, 1965 INVENTORS ROBERT K. VAN AUSDAL DONALD V. HEALY JOHN P. ANDERSON June 25. 1968 VAN AUSDAL ET AL 3,389,719

ELECTROHYDRAULIC SERVOVALVE HAVING MINIMUM FLOW DURING QUIESCENT OPERATION Filed Oct. 18, 1965 2 Sheets-Sheet 2 Fig. 2

DIFF CHAMBER PRESSURE- PSI Fig. 4

IN VEN TORS ROBERT K VAN AUSDAL 400 DONALD V. HEALY JOHN P. ANDERSO United States Patent "ice $389,719 ELECTROHYDRAULIC SERVOVALVE HAV- ING MINIMUM FLOW DURING QUIES- CENT OPERATION Robert K. Van Ausdal, La Crescenta, Donald V. Healy,

Newhall, and John P. Anderson, Van Nuys, Caliti, assignors to The Bendix Corporation, a corporation of Delaware Filed Oct. 18, 1965, Ser. No. 497,196 9 Claims. (Cl. 137--625.61)

ABSTRACT OF THE DISCLGSURE An eleclrohydraulic servovalve including a torque motor with an armature and a flapper member attached at its center and movable with the armature for varying the flow of control fluid to one side or the other of a spool valve which, in turn, controls the application of fluid pres sure to a hydraulic motor. The flapper member is attached to a shaft rotatable along the axis of rotation of the armature. A spring-loaded poppet valve is attached at each end of the flapper member to block the flow of fluid to the spool valve in the absence of a control signal such that there will be minimum flow and minimum noise genera'ed when there is no input signal or an input signal below a specific level.

This invention relates to electrohydraulic servovalves, and more particularly to a dry type valve in which the electrical parts are sealed from the actuating fluid and in which there is a minimum of flow from the control orifices during quiescent or no-input signal operation.

The dry coil type of servovalve has been proven in operation for a number of years and is quite successful in performing tasks in many fields. This type of servovalve, which is shown in Patent No. 3,095,902 issued to Donald V. Healy and assigned to the assignor of the present application, typically includes a pair of nozzles or control orifices directing a continuing flow of actuating fluid against a flapper valve member which is moved toward one such orifice and away from the other to produce pressure variations upstream of the orifices for moving a control member. Recently it has been found desirable to produce control valves of the closed center type wherein there is no flow from the control orifices unless there is a signal to the coil tending to move the flapper valve. This desire to operate with zero quiescent leakage has resulted in introducing some new problems into the design of servovalves. Obviously, there must be effective sealing means between the flapper and the orifices. Applicants have found that such sealing means tends to substantially increase the stiffness of the mecanical parts, this requiring increasing electrical energy for a given displacement of the flapper. Another difficulty which manifested itself was an unsatisfactory lack of response or dead band in the transfer characteristic around the point of zero electrical input. This dead band results in considerable hysteresis in the system, and is generally undesirable. It is therefore an object of the present in vention to provide a dry coil electrohydraulic servovalve which operates with essentially zero quiescent leakage at the control orifices.

It is another object of the present invention to provide an electrohydraulic servovalve meeting the above objective in which the mechanical operation is reasonably flexible such that electrical input signals required for satisfactory operation are of generally comparable magnitude to those used in conventional servovalves of comparable size and rating.

It is a further object of the present invention to pro- 3,339,739 Patented June 25, 1968 vide an electrohydraulic servovalve meeting the above objectives and in which the dead band problem referred to above is substantially eliminated.

Other objects and advantages will become apparent from consideration of the following specification taken in connection with the accompanying drawings, in which:

FIGURE 1 is an isometric projection, partially cut away to show details of an electrohydraulic servovalve incorporating our invention.

FIGURE 2 is a sectional drawing of a valve structure shown schematically in FIGURE 1.

FIGURE 3 is a graph showing the transfer characteristic of a servovalve such as that shown in FIGURE 1 when no means was employed to correct for the dead band around the point of zero electrical input.

FIGURE 4 is a graph like that of FIGURE 3, but showing substantially improved response in the region of zero electrical input.

Referring now to FIGURE 1, the electric motor assembly, shown generally at numeral 10, includes a pair of bar magnets 12 and 14 mounted to introduce flux into a pair of ferromagnetic frame members 16. The frame members 16 include arms 18 terminating in faces 20 and 22 opposite the ends of an armature 24 mounted for rotation on a support 26. Armature 24 passes through central openings in a pair of coils 28 and 30 which are held in place wilhin the enclosure made by the frame member 16.

The armature 24 is lightly held midway between the op posing faces of the magnetic frame members 16 for movemeat from the mid-position under the control of changes in flux in the magnetic system resulting from passage of current through the coils 28 and 39.

The support 26 for armature 24 is itself carried by an upstanding tubular support, hereinafter designated the torque tube 32, including an enlarged collar portio. 34 which is sealed to an opening in a wall 36 in the housing of the hydraulic assembly. The upper end of torque tube 32 is closed by a central shaft member 38 sealed therein as by brazing or an interference fit. The seal between shaft 38 and the torque tube 32 must be adequate to at least resist the drain or return hydraulic pressure and to prevent the ingress of hydraulic fluid from the hydraulic assembly to the motor assembly 19. The tube 32 is preferably of a spring material, such as Inconel X or beryllium copper, having a thin wall in the order of 0.006 to allow rotational movement of the armature 24 around the axis of the tube 32 by twisting of the tube 32. The rotation of the armature 24 is transmitted through shaft 38 into the hydraulic housing where it is attached to an elongated motion-transmitting member or bar 40, including a pair of end portions 42 and 44. Each of these end portions actuates an identical poppet valve mechanism, only one of which is shown in detail in FIGURE 1. Shaft 38- includes a notched section 39 of reduced cross-sectional area whose function is to reduce resistance to forces tending to bend the shaft. The purpose of this is discussed below.

The hydraulic system used in combination with the motor assembly it provides the power for controlling a double-acting piston 5% which reciprocates within a cylinder 52. Control pressure to one side or the other of piston is communicated through a pair of conduits 54 and 56 which communicate with a cylinder 58 containing a spool valve member 60 urged toward a centered position by means of a pair of springs 62 and Hydraulic fluid under pressure is supplied to spool valve cylinder 53 from a. reservoir 66 by means of a pump 68 which pumps the fluid through a conduit 70, which may contain a filter 72, and to a pair of inlet passageways 74 and 76. Return fluid flows from the low pressure side of piston 50 through either of conduits 54 or 56, past spool 60 and into the reservoir 66 via a conduit 78. Also connected to conduit is a passage which supplies hydraulic fluid under pressure to each of two identical branches of the servo system, one containing a valve 8?. in communication with a conduit 84 which is connected at one end with one end of the spool valve cylinder 58 and at its opposite end with a reservoir 86 which includes a filtering screen 88 and from thence through a short channel 90 to a chamber 92 containing the poppet valve mechanism referred to above. The other branch contains a valve 94 which communicates with a conduit 96 connected at one end with the other end of the spool valve cylinder 58 and at its opposite end with a reservoir 98 having a filtering screen 186 and from thence through a channel 102 to a chamber 104 containing another poppet valve mechanism.

Since the poppet valve mechanisms in chambers 92 and 104 are identical, only that appearing in chamber 92 is shown in detail. The end section 44 of bar 40 carries a contact member 196 which is somewhat pointed on one end to make a low friction contact with a spring retainer 108. A spring bears against retainer 188, and the force exerted thereby is adjustable by means of a second spring retainer member 112 which is threadedly engaged with a sleeve 114 held in chamber 92. The opposite end of contact member 106 is fiat and acts against the somewhat rounded contact surface of a poppet valve member 116. The tapered valve surface of poppet 116 seats against an annular seat on a member 118 aflixed within a sleeve member 120 held stationary within chamber 92. Poppet 116 controls communication between a passageway within seat member 118 and a port 122 which communicates with an annular chamber 124. A conduit 126 connects chamber 124 with the low pressure or return side of the hydraulic system.

The valves shown schematically in FIGURE 1 as valves 82 and 94 are identical and are actually constructed as shown in FIGURE 2. A housing 130 includes a cylindrical sleeve member 132 having a reduced diameter portion 134 which cooperates with an interior wall of housing 130 to form an annular chamber 136. Ports 138 in portion 134 provide communication between the interior of member 132 and chamber 136 and from thence to conduit 84. Axially slidable within sleeve 132 is a poppet valve member 140 which is urged against a seat 142 by means of a spring 144. Poppet 146 includes a small orifice 146 and a plurality of larger ports 148. A larger orifice is located upstream of seat 142 and is formed in a plate 152 secured between seat member 142 and an annular spacer 154.

As indicated above, the purpose of the servo valve structure is to control the flow of hydraulic fluid to one side of actuating piston 59 in cylinder 52. As described, it will be recognized that the opposite sides of the hydraulic system are symmetrical; therefore, if bar 40 is centered and both of the poppet valve members in chambers 92 and 104 are closed and hydraulic fluid is supplied from pump 68 through conduit 70 and filter 72 to conduits 74 and '76 and through passageway 80 and valves 82 and 94 to conduits 84 and 96, the fluid pressures acting against the opposite sides of spool valve 60 are equal and spool 60 is maintained in its centered position. So positioned, it blocks flow from passages 74 and 76 as Well as from passages 54 and 56. The fluid pressure on opposite sides of piston 51 remains stabilized, and piston 50 does not move.

Assume now that a control signal is supplied to the windings 28 and 30 which causes armature 24 to move toward pole face 22 and away from pole face 20. The resulting torque is transmitted through shaft 318 to bar 40. Since bar 40 is effectively held stationary at end section 42, it tends to rotate around this end as it permits pressure in chamber 118 to open poppet valve member 116 against the force of spring 110. This requires that the lower end of shaft 38 be translated in the same direction as end section 44 and approximately one-half the distance, thus requiring that shaft 38 bend to make this possible. It has been found that a substantial amount of electrical power is required to effect this bending when the prior art type of shaft is utilized. In order to permit the shaft 38 to bend while maintaining a substantial level of torsional rigidity, its cross-section has been reduced to leave a thin section 39 which offers little resistance to forces tending to translate the bottom of shaft 38 along a line essentially parallel to the direction of movement of the ends of bar 40. Actually, this movement is somewhat arcuate, but the travel is sufficiently short that the movement may be treated as linear. Section 39 still offers substantial resistance to torsional forces or to bending forces acting perpendicular to the direction of movement of the ends of bar 40.

Movement of poppet valve member 116 off its seat permits fluid to flow from the interior of seat member 118, through port 122, into chamber 124 and through conduit 126 to the low pressure side of the hydraulic system. This causes a reduction in the fluid pressure acting against the right end of spool valve 60, causing it to move toward the right against the pressure spring 64 and communicating high pressure fluid in passage 76 with conduit 56 and the right side of piston 50. Simultaneously, the left side of piston 50 is placed in communication with the low pressure side of the hydraulic system through conduits 54 and 78, thus causing piston 59 to move toward the left. Introduction of the opposite signal into windings 28 and 30 causes movement of bar 40 around its end 44 and opening of the poppet valve in chamber 104. This, of course, results in reducing the pressure acting against the left end of spool 60, causing it to move to the left with the result that high pressure fluid is supplied from passage 74 through passage 54 to the left side of piston 50. At the same time, conduits 56 and 78 are placed in communication, which permits fluid on the right side of piston 50 to be exhausted into reservoir 66.

In the conventional type of servovalve in which constant leakage is permitted in the first stage, valves 82 and 24 are replaced by simple fixed restrictions or bleeds such as that in plate 152 (FIGURE 2). In the closed-center design, however, it was found that when the spring loads on the poppets were made heavy enough to assure minimum leakage, the dead band around null or zero electrical input became excessive. This characteristic is shown in FIGURE 3 where the electrical input is plotted on the horizontal axis and the differential chamber pressure on the vertical axis. The electrical input will be seen to vary over a range from approximately 10 to +10 milliamperes with only a negligible change in differential chamber pressure. The next increment of -10 ma. or +10 ma. produces substantial differential chamber pressures. It was found that a great improvement in dead band characteristics could be obtained if a variable orifice of much smaller effective area were placed in series with the normal fixed restriction. With reference to FIGURE 2, when the poppet member 140 is seated, the initial pressure gain upon opening of one of the first stage poppets is a function almost solely of the pressure drop across the smaller orifices 146. After this pressure drop reaches a predetermined value, as established by the loading of spring 144, piston 140 leaves seat 142, permitting flow through ports 148 and 138 and into conduit 84 which causes the rate of pressure build-up to be reduced. Thus, the small orifice 146 is bypassed and orifice 150 becomes the effective metering orifice. The improved operation with the valve structure above described is shown in FIGURE 4. It will be Observed that the slope is very steep across null, indicating substantial gain in the differential chamber pressure with small changes in input current. This rate becomes somewhat less at greater values of input current, reflecting the transition to the fixed orifice when the poppet valve 140 opens. For greater positive or negative input currents, the characteristic is essentially the same as that of FIGURE 3. The slope through neutral is basically dependent on the ratio of the area of orifice 146 to the effective area of the port defined by poppet 116 and seat 118. The point of transition from the steep slope to the normal slope is determined by the force exerted by spring 144 against piston 140 and which may be varied by adding to or subtracting from a number .of shims 156.

While only one embodiment is shown and described herein it is recognized that modifications may be made, and we do not intend to be limited to the structure described, or other than as defined by the appended claims.

We claim:

1. In an electrohydraulic servovalve including a coil for receiving electrical signals;

an armature around which said coil extends for magnetizing said armature when a signal is received by said coil;

two pairs of magnetic poles of opposite polarity, one pair extending into close proximity to said armature beyond one end of said coil and the other pair extending into close proximity to said armature at the opposite end of said coil;

a hydraulic system including a housing and a source of actuating fluid;

a flow control system within said housing including a pair of substantially parallel passageways connected to said source, valve means for closing each of said passageways, resilient means urging both said valve means in a closing direction, and an elongated motion-transmitting member connected to said valve means,

means sea-ling said hydraulic system from said coil and said armature comprising a non-rigid tube having one end sealed to said housing and a shaft sealed to the opposite end of said tube and extending through said tube, said shaft being coupled approximately midway along said motion-transmitting member to transmit torque from said armature to said member to open the opposite of said valve means, said shaft including a section having a reduced cross-sectional area for reducing its resistance to bending forces,

a valve cylinder connected to said source,

a spool valve positioned within said cylinder and means communicating the fluid pressure in said passageways to opposite ends of said spool valve to vary the position of said spool.

2. In an electrohydraulic servovalve including a coil for receiving electrical signal-s;

an armature around which said coil extends for magnetizing said armature when a signal is received by said coil;

two pairs of magnetic poles of opposite polarity, one pair extending into close proximity to said armature beyond one end of said coil and the other pair extending into close proximity to said armature at the opposite end of said coil;

a hydraulic system including a housing and a source of actuating fluid;

a flow-control system Within said housing including a pair of substantially parallel passageways connected to said source, flow restriction means in each of said passageways, valve means for closing each of said passageways, resilient means maintaining both said valve means normally closed, and an elongated motion-transmitting member connected to said valve means,

means sealing said hydraulic system from said coil and said armature comprising a non-rigid tube having one end sealed to said housing and a shaft sealed to the opposite end of said tube and extending through said tube, said shaft being coupled approximately midway along said motion-transmitting member to transmit torque from said armature to said member to cause said member to efiectively be rotated around one of said valve means and to open one of said valve means, said shaft including a section having a reduced cross-sectional area for reducing its resistance to bending forces resulting from said rotation,

a valve cylinder connected to said source,

a spool valve positioned within said cylinder and means communicating the fluid pressure in said passageways to opposite ends of said spool valve to vary the position of said spool.

3. An electrohydraulic servovalve as set forth in claim 2 wherein each of said flow-restriction means includes, in series, means defining a primary orifice and a piston carrying a poppet valve including an orifice of smaller effective area and means for urging said poppet valve in a closing direction such that when said poppet valve is closed, the effective area of said restriction is defined by said poppet valve orifice and when said poppet valve is opened in response to pressure differentials across said piston exceeding a predetermined value, the effective area of said restriction is defined by said primary orifice.

4. A servovalve comprising:

a hydraulic housing and a source of actuating fluid connected thereto;

a flow-control system within said housing including a pair of substantially parallel passageways connected to said source, poppet valves for closing both of said passageways, resilient means maintaining both said poppet valves normally closed, and an elongated motion-transmitting member connected to said poppet valves;

a motor compartment;

motor means within said compartment,

means sealing said flow-control system from said motor means comprising a non-rigid tube having one end selaed to said housing and a shaft sealed to the opposite end of said tube and extending through said tube, said shaft being coupled approximately midway along said motion-transmitting member to transmit torque from said motor means to said member to open one of said poppet valves,

said tube and shaft supporting said motion-transmitting member for permitting simultaneous opening of said poppet valves in response to increases in pressure from said source.

5. A servovalve as set forth in claim 4 wherein said shaft includes a section having a reduced cross-sectional area for reducing its resistance to bending forces.

6. A servovalve comprising:

a hydraulic housing and a source of actuating fluid connected thereto;

a flow-control system Within said housing including a pair of substantially parallel passageways connected to said sour-cc, valve means for closing both of said passageways and an elongated motion-transmitting mernber connected to said valve means;

a motor compartment and motor means in said compartment;

means sealing said flow-control system from said motor means comprising a non-rigid tube having one end sealed to said housing and a shaft sealed to the opposite end of said tube and extending through said tube, said shaft being coupled to said motion-transmitting member to transmit torque from said motor means to said member to open one of said valve means in response to a signal to said motor means,

said tube and shaft supporting said motion-transmitting member for permitting simultaneous opening of both of said valve means in response to increases in pressure from said source.

7. A servovalve as set forth in claim 6 wherein said shaft includes a section having a reduced cross-sectional area for reducing its resistance to bending forces.

8. A servovalve comprising:

a hydraulic housing and a source of actuating fluid connected thereto;

a flow-control system within said housing including a pair of substantially parallel passageways connected to said source, valve means for closing both of said passageways and an elongated motion-transmitting member connected to said valve means;

a motor compartment and motor means in said compartment;

means sealing said flow-control system from said motor means comprising a non-rigid tube having one end sealed to said housing and a shaft sealed to the opposite end of said tube and extending through said tube, said shaft being coupled to said motion-transmitting member to transmit torque from said motor means to said member to open one of said valve means in response to a signal to said motor means,

said tube and shaft supporting said motion-transmitting member for permitting simultaneous opening of both of said valve means in response to increases in pressure from said source,

a valve cylinder connected to said source,

a spool valve positioned within said cylinder,

a pair of springs urging said spool valve toward a centered position in said cylinder and conduit means communnicating the fluid pressure in said passageways to opposite ends of said spool valve to vary its position in said cylinder.

9. A servovalve comprising:

a hydraulic housing and a source of actuating fluid connected thereto;

- a flow-control system within said housing including a pair of substantially parallel passageways connected to said source, poppet valve for closing both of said passageways, resilient means urging both of said poppet valves in a closing direction, and an elongated motion-transmitting member connected to said poppet valves;

a motor compartment and motor means within said compartment;

means sealing said flow-control means from said motor means comprising a non-rigid tube having one end sealed to said housing and a shaft sealed to the opposite end of said tube and extending through said tube, said shaft being coupled approximately midway along said motion-transmitting member to transmit torque from said motor means to said member to cause said member to rotate around one of said poppet valves and open the other of said poppet valves, said shaft including a section having a reduced cross-section for reducing its resistance to bending forces resulting from said rotation.

No references cited.

25 M. CARY NELSON, Primary Examiner.

R. J. MILLER, Assistant Examiner. 

